Dibenzylidene sorbitol, dibenzylidene xylitol and nuclear substitution compounds thereof have unique properties as additives and are used, for example, as transparency imparting agents for polypropylene resins, anti-rutting agents for asphalt pavements, precipitation preventing agents for preparing and storing coal-and-oil mixture fuels, flowability improving agents for PVC paste sols, flowability improving agents for FRP, excipients for pharmaceuticals, cosmetics and the like, marking agents, fuel solidifying agents, etc. Among the benzylidene sorbitol, benzylidene xylitol and nuclear substitution compounds thereof, especially the 1,3- and 2,4-substituted compounds represented by the following structural formula have the properties useful as such additives. ##STR1## wherein R' is an alkyl group, alkoxy group, halogen atom or nitro group, and p is an integer of 1 or 0.
Accordingly it is important to prepare such 1,3- and 2,4-substituted compounds in high yields and with high selectivities.
Dibenzylidene sorbitol is usually produced by the dehydration condensation reaction of sorbitol and benzaldehyde. Processes have been developed in recent years for carrying out this reaction in the form of a slurry with use of cyclohexane and/or a saturated hydrocarbon having 6 to 10 carbon atoms in a large amount as the reaction medium (Published Examined Japanese Patent Applications No. 43748/1973 and No. 14758/1974). These processes utilize the properties of the reaction medium that it is azeotropic with water and free of gelation by the resulting benzilidene sorbitol. More specifically when the reaction medium of cyclohexane and/or saturated hydrocarbon with 6 to 10 carbon atoms is used, the water resulting from the reaction is continuously withdrawn from the system by an azeotropic phenomenon to promote the dehydration reaction, while the product separates out and disperses in the reaction medium in the form of pearl-like particles without gelling or solidifying the medium, permitting the reaction to proceed in the state of a slurry having a low viscosity. Accordingly the reaction can be carried out efficiently in a reactor equipped with a usual impeller mixer while assuring good dispersion of the charge, consequently giving the desired product in a yield of at least about 70% within a relatively short period of time, i.e. about 5 to 7 hours. For the reaction medium to be serviceable as a dispersing agent in the processes, the medium is limited to cyclohexane and/or saturated hydrocarbons having 6 to 10 carbon atoms which will not be gelled or solidified by the resulting benzylidene sorbitol. Furthermore it is critical that the medium be used in a large amount to maintain the reaction system in the form of a slurry as mentioned above.
Our research has revealed the totally unexpected fact that although the reaction system becomes a gel to solid phase when cyclohexane and/or saturated hydrocarbon are/is used in a small amount, the reaction system, if forcibly agitated, affords in a high yield and with a high selectivity the desired compound, especially 1,3- and 2,4-substituted compounds having the above-mentioned unique properties as additives. More specifically when a system comprising sorbitol or xylitol, a benzaldehyde or an alkyl acetal derivative thereof, a catalyst and a water-soluble polar solvent according to the conventional process is subjected to reaction with addition of a small amount of cyclohexane or the like, the resulting dibenzylidene sorbitols or dibenzylidene xylitols separate out as crystals while gelling or solidifying the reaction system to render the cyclohexane or the like no longer serviceable as the dispersing medium as used in the conventional process. Surprisingly, however, when the reaction system in the form of a gel to solid phase is forcibly agitated, the system gives in high yields and with high selectivities dibenzylidene sorbitols or dibenzylidene xylitols which have the aforementioned unique properties as additives. Our research has further revealed that such a result is achieved not only when using cyclohexane, saturated hydrocarbons, etc. which will not be gelled by dibenzylidene sorbitols or dibenzylidene xylitols but also when using hydrophobic solvents alike which will inherently be gelled or solidified by such compounds. The present invention has been accomplished based on such novel findings.